The present invention relates to a pneumatic tire, more particularly to a bead structure suitable for heavy duty tires.
In Japanese Patent Application Publication No. 2005-178618, a heavy duty tire is disclosed, wherein the edges (ce) of a carcass ply (c) are each winded almost once around a bead core (b) as shown in FIG. 5 in contrast to a conventional structure as shown in FIG. 6 wherein the edges of a carcass ply are each turned up around a bead core so as to extend radially outwards.
In the case that the carcass ply edge is winded around the bead core, when the carcass ply main portion (cm) is pulled radially outwards due to the tire inflation, axially outward deflections of the sidewall portion and the like, the bead core (b) is subjected to rotational deformations around its central axis. As a result, the contact pressure between the bead bottom (bb) and the bead seat (bs) of the wheel rim on which the tire is mounted is increased locally in a heel-side region (y) than a toe-side region of the bead bottom surface. Therefore, the bead bottom surface is hardened unevenly due to the locally increased pressure and heat during long time use. More specifically, the hardness of the bead bottom surface becomes high on the heel side than the toe side and as a result cracks are liable to occur. If such local hardening and/or cracks are occurred in the bead portions, the tire can not be reused by retreading.